More experienced users can correct me, but my short answer:
Build in plugin for Multiplayer based on Host and Peer, that is good for small games but have its own limitations.
The other unlimited option is to build everything by yourself: server, comunication, jsons, ajaxes etc... - and that is only dependence on your developer and programming skills. You can achieve great games, but again, its only depend on you and its far from being easy.
My advice, if you only starting with construct, don't start with big multiplayer game as your first project, because you'll get frustrated and wanna quit. And it can be unfortunate because construct is really good tool that worth exploring. Better to try start making platformer game and later making it multiplayer, rather than making straight multiplayer game.